The present invention concerns a method for reducing hydrocarbon emissions by use of adsorbent materials. In particular, the invention concerns the use of polymeric absorbents for reducing hydrocarbon emissions from fuel reservoirs.
The United States and other countries have enacted regulations requiring motor vehicle manufacturers to develop systems limiting emissions of hydrocarbon fuels to certain prescribed levels. Fuel storage tanks employed in motor vehicles inevitably develop elevated tank pressures due to vaporization of the fuel into a void space created between the surface of the fuel and top of the tank. The vapor formed is actually a mixture of air and hydrocarbon fuel vapors. Vaporization of the fuel occurs primarily due to heating of the fuel, such as when a motor vehicle is subjected to atmospheric temperature fluctuations during the course of a day. As a result of such pressure buildups, storage tanks are vented for safety purposes.
To reduce hydrocarbon emissions into the atmosphere, vehicles have been designed with systems that vent such storage tanks through a canister containing activated charcoal. The hydrocarbons contained in the air/fuel vapor mixture are then adsorbed to some extent onto the activated charcoal and, thus, are not expelled into the atmosphere. When the engine of the motor vehicle is subsequently operated vacuum pressure from the vehicle's engine manifold is used to draw air through the activated charcoal to desorb the hydrocarbons retained thereon. The desorbed hydrocarbons are then directed to the vehicle's engine where they are subsequently mixed with air and combusted. Examples of such systems are seen in U.S. Pat. Nos. 3,093,124; 391919587; 3,352,294; and 3,393,669. U.S. Pat. No. 4,906,263 discloses use of a polyurethane foam support matrix impregnated with granules of adsorbent materials, such as activated charcoal, to remove pollutants and gases from fluid streams.
Due to enactment of more stringent environmental regulations requiring even greater reductions in fuel vapor emissions, the use of activated carbon in the above-described systems is becoming less desirable due to its low capacity for adsorbing fuel vapors. To meet the new regulatory standards, it may be necessary to employ larger amounts of activated charcoal in such canisters, in some cases as much as four times the amount presently employed. As the space available within motor vehicles for such canisters is limited, simply using more activated charcoal and a larger canister is not feasible. The use of larger amounts of adsorbent would increase vehicle weight, which undesirably reduces fuel economy. Any design changes associated with the use of a larger canister may also require expensive re-tooling of equipment used in the manufacturing process.
Certain types of polymeric adsorbents have been previously proposed for fuel vapor recovery systems, but the capacity of such adsorbents has been, at best, only comparable to that of activated carbon. For example, U.S. Pat. No. 3,844,739 discloses use of a dual-bed sorbent system wherein one bed is a styrene-divinylbenzene copolymer having from 17 to 75 weight percent divinylbenzene and the second bed is of activated carbon. U.S. Pat. Nos. 3,798,826 and 3,805,493 disclose use of adsorbents based on acrylic and nonpolar aromatic polymers. U.S. Pat. No. 4,863,494 discloses the use of porous divinylbenzene polymers.
In addition to emissions of fuel vapors from motor vehicles, the above-described problem is similarly applicable to fuel storage tanks or reservoirs used to hold or transport fuels. Such reservoirs are typically vented, which also presents the possibility for release of fuel vapors. It would be desirable to employ an improved adsorbent for reducing emissions of fuel vapors from these reservoirs as well.
As can be seen, what is needed is a method to reduce fuel vapors that employs an adsorbent material having a relatively high capacity for retaining hydrocarbons. Such an adsorbent would be a suitable replacement material for activated carbon in a wide variety of systems designed to reduce fuel vapor emissions, and in particular, fuel vapor emissions associated with motor vehicles.